The process intensification of biological hydrogen production by Escherichia coli HD701

Abstract

Hydrogen is seen as a potential fuel for the future; its choice is driven by the increasing awareness of the necessity for clean fuel. Together with the simultaneous development of “green technologies” and sustainable development, a current goal is to convert waste to energy or to create energy from a renewable resource. Biological processing [of renewables] or bioremediation of waste to create hydrogen as a product fulfils this goal and, as such, is widely researched. In this work, an already established process, using a hydrogenase up‐regulated strain ‐ was characterised and the important process parameters were established. This bacterial strain has the potential for industrial‐scale hydrogen production from, for example, waste sugars. Previous work, repeated here, showed that hydrogen could be generated by E. coli HD701 using a two‐phase process (growth in shake flasks, followed by hydrogen production within a bioreactor). Ideally a commercial process would need to be in a single vessel (bioreactor), which therefore resulted in this investigation of the scale‐up of twophase fermentations to 5 L stirred tank bioreactors. Within the initial two‐phase process, shake flask growth in 2 L shake flasks (employing a 50% working volume) achieved a dry cell weight of 1.33 +- 0.1 mg mL‐1 which then, when transferred to a 5 L bioreactor (containing 2 L of culture and 2 L of hydrogen production substrate), achieved a maximum hydrogen production rate of (200 mL h‐1) 150 mL g(dcw)‐1 h‐1. The first step in scale‐up was to simply transfer the process to a bioreactor and see the effect it had on hydrogen production. This approach did not yield any hydrogen and therefore consequent experimentation sought to see if the hydrogen production was growth phase dependant. However all phases of growth evolved no hydrogen upon the addition of substrate. The next approach was to take the conclusion drawn from a literature survey that showed a need for microaerobiosis or anaerobiosis during growth (for mixed acid fermentation to occur) along with a high formate concentration necessary for the transcription of the FHL complex (the hydrogen gas evolving enzyme). For this reason the KLa from the initial shake flask growth (calculated from literature correlations) was applied to the bioreactor. Experiments used to simulate the shake flask mass transfer coefficient (kLa) in a bioreactor did not generate hydrogen; the physical system within the shake flask used for growth in the initial process allows for this to occur, but the consequent process change to a bioreactor did not. This inability to produce hydrogen was concluded to be due to the lack of microaerobiosis/anaerobiosis required for mixed acid fermentation (the metabolic precursor to hydrogen production). The criterion of KLa was inappropriate for scale up in this case due to the physical differences between the shake flask and the bioreactor, as the oxygen transfer within the shake flask is not limited to transfer between the liquid and gas phase (the effect of transfer across the shake flask closure must be considered). This fact led to the novel use of gas blending for dissolved oxygen tension control. Gas blending was used in a bioreactor to track the changes observed during growth in the shake flask. This created a process that mirrored the shake flask in both growth and hydrogen production. The outcome was a dry cell weight of 1.34 +- 0.02 mg mL‐1 and a maximum hydrogen production rate of 200 mL h‐1 i.e. 150 mL g(dcw)‐1 h‐1, exhibiting almost identical process results to the two‐stage process. This characterisation reinforced the necessity for microaerobiosis during growth to allow subsequent post‐growth hydrogen production. Microaerobiosis in the latter stages of growth allows mixed acid fermentation to occur, which was found to be essential for hydrogen production. Process intensification took place by increasing cell density. This was achieved by increasing the medium concentration, then by changing the medium (two differing fed batch media were chosen; each medium used was experimentally linked with multiple feeds) and finally by utilising the novel technique of combining gas blending with fed batch cultivation to ensure microaerobiosis during growth. This, along with the use of a low (\(\mu\)=0.05 h‐1) growth rate for feed calculation, led to an eight‐fold increase in cell density. The low growth rate was employed to reduce inhibitory acetate formation while the multiple feeds were used to investigate nitrate depletion. The maximum increase in cell density led to a hydrogen evolution rate of 1800 mL h‐1, thus producing hydrogen that could be converted into energy at a rate eleven‐fold greater than the rate at which it consumed energy for agitation.

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